1. Field of the Invention
The present invention relates to a velocity-modulation transistor having a high-speed channel and a low-speed channel.
2. Description of the Prior Art
The channel conductance G of a field-effect transistor is given, in general, by the following equation: EQU G=qn .mu.eff (1)
where q is the charge on carriers, n is the carrier concentration, and .mu.eff is the effective carrier mobility of the carriers.
Generally, the channel conductance G is increased or decreased by changing a voltage applied to a gate of a transistor, to perform operations as a device such as an on-off operation of a source-drain current. The change in channel conductance .DELTA.G relative to the change in gate bias .DELTA.Vg is given by the following equation: EQU .DELTA.G=q.DELTA.n .mu.eff+qn.DELTA..mu.eff (2)
In a conventional field-effect transistor, the change in carrier concentration .DELTA.n in the first term of the equation (2) contributes to .DELTA.G. Consequently, the operating speed of a device depends on the charging and discharging time of carriers. Accordingly, a high speed of less than 1 psec cannot be expected.
Therefore, a velocity-modulation transistor for operating a device by the second term of the above described equation (2), that is, the change in mobility .DELTA..mu.eff has been proposed in a document entitled "Velocity-Modulation Transistor", Japanese Journal Applied Physics, Vol. 21, No. Jun. 6, 1982, pp. 381-383. (A copy of this proposal accompanies this application).
The device proposed herein comprises a channel A (ch. A) in which carriers can move at high speed and a channel B (ch. B) in which carriers can move at low speed. The two channels have symmetrical potential shapes. Two gates, that is, a gate A and a back gate B are respectively provided in an upper part of the channel A (ch. A) and in a lower part of the channel B (ch. B) in order for the carriers to move between the channels. In this device, an operation of performing velocity modulation is obtained by changing a gate bias to change a channel in which carriers flow. In this case, the operating speed of the device is determined only by the time required for the carriers to move between the channels, thereby allowing a switching operation with a high speed of less than 1 psec.
However, the above described construction has the following problems.
First, the difference in speed between the channels A and B is caused by the difference in the amount of impurities in the channels. However, the scattering of the carriers due to the impurities is effective only at low temperatures, that is, at temperatures below the temperature of liquid nitrogen. Accordingly, there is little difference in speed at room temperature, so that the device does not operate at room temperature.
Second, the two channels have symmetrical potential shapes. Accordingly, the two gates, that is, the gate A and the back gate B are respectively required in the upper part of the channel A (ch. A) and in the lower part of the channel B (ch. B) in order for the carrier to move between the channels.
Third, when the device is on, that is, a steady-state current IL flows in the device, the device is in exactly the same internal state as that of an ordinary high electron mobility transistor (referred to as HEMT hereinafter), so that the power consumption thereof is the same as that of the ordinary HEMT. Consequently, a velocity-modulation transistor is not currently realized in the above described construction.